1. Field of the Invention
The present invention relates to an improvement in a fixing device for an image forming apparatus which is used to fix a toner image on an image forming medium such as a sheet of paper passed between a heating roller and a supporting roller.
2. Description of the Prior Art
Generally, in an image forming apparatus, a sheet of paper, such as the image forming medium onto which a toner image has been transferred, is conveyed to a fixing device after having been peeled from a photosensitive drum, and a fixing operation is carried out. In order to fix the toner image on the paper, the fixing device comprises a roller pair including a fixing roller and a pressing roller forming a nip portion therebetween. Hitherto, when the paper passed through the roller pair, creases occurred in the paper caused by the roller pair.
Utility Model Application Laid Open under No. 123955/1980, Utility Model Application Publication Nos. 9806/1985 and 14284/1985 in Japan are directed to solve this problem. In these applications, a method of preventing paper creases calls for guiding the paper which had been fed to the entry to the roller pair and, at the same time, providing a paper guide plate with its center in the lengthwise direction of the roller pair of a higher convex shape than both side edges. The paper conveyed on the paper guide plate was fed to the roller pair in such a way that both edges were lower than the center.
However, even by this method, in cases where the leading edge of the paper curls, as shown in FIG. 10A, it is not often that both side edges of the paper fall lower than the center portion when following the paper guide plate under the paper's own weight alone. In particular, in cases of copying on both sides of the paper or multiple copying on the same side of the paper, in which a toner image is fixed a second time onto the paper which has already once been developed and fixed, a curl will occur at the leading edge of the paper and the paper will not be fed correctly to the roller pair for the second copying operation. Therefore, there is the problem of creases occurring and, in the worst case, of the occurrence of paper jamming at the roller pair.